This invention relates to a method and apparatus for detecting an edit point on a record medium and, more particularly, to such a method and apparatus wherein digitized information is recorded in data tracks on the record medium and a control signal including an address signal for identifying recurring intervals on the record medium is recorded in a control track.
Recently, digital information has been recorded directly on a record medium, such as magnetic tape. Such digital recording techniques have been extended into fields which, heretofore, had been reserved solely for analog recording. For example, audio signals now are digitized as, for example, PCM signals, and the digitized audio signals are recorded. Typical PCM audio recorders are described in, for example, U.S. Pat. Nos. 4,211,997 and 4,145,683.
The use of digital techniques for recording audio information generally has enhanced the quality of the reproduced audio sounds. Furthermore, desired portions of the digitized information may be easily and precisely accessed. This facilitates the electronic editing of a digitized audio record.
However, whereas loss of information in an analog record generally may be concealed, or masked, so as to be generally unnoticed, a comparable loss of digitized information due to, for example, drop-out, interference, fingerprints on the record medium, and the like, may be highly pronounced. Accordingly, to minimize such disturbances, the digitized information generally is recorded in an error-correction code. One recent error correcting code which has been developed and which is particularly useful in recovering digitized information that may be subjected to such drop-out, interference, and the like is the so-called cross-interleave error correction code described in, for example, U.S. application Ser. No. 218,256, filed Dec. 19, 1980. Other error correction encoding techniques also are known, such as described in U.S. Ser. No. 195,625, filed Oct. 9, 1980. In accordance with such error correction codes, a number of digital words, such as PCM words, are time-interleaved with each other to form data blocks. Successive data blocks are recorded in one or more data tracks on, for example, magnetic tape. If a separate control track also is recorded, address signals may be recorded in that control track so as to identify predetermined intervals, such as "sector intervals". By addressing the appropriate sector interval, the data blocks recorded therein may be readily accessed. This feature is advantageous when it is desired to edit the information in certain sector intervals. When using this technique, it is best to increment the address signals, known as sector address signals, consecutively from one interval to the next.
While a magnetic tape which is recorded in the aforementioned manner, viz. having a plurality of data tracks and a single control track recorded thereon, may admit of relatively smooth electronic editing such that the various edit points are not readily noticed, such digitally recorded tapes are less advantageously used in so-called "splice editing". In splice editing, two separate magnetic tapes are physically joined, or spliced, such that the information recorded on one tape physically follows the information that is recorded on the other. It is expected that, on either side of the "splice" or joint, the digitally recorded information may be subjected to error. In particular, a discontinuity is present in the reproduced digitized information when the splice point is reached. To prevent this discontinuity from seriously interfering with the audio signals which ultimately are reproduced from the digital recording, techniques such as the so-called cross-fading technique, muting, and the like, generally are employed. These techniques are described in, for example, aforementioned U.S. Ser. No. 195,625 and also in U.S. Ser. No. 116,401, filed Jan. 29, 1980.
When minimizing the effects caused by the discontinuity at the splice edit point, it is important to detect when this point has been reached. One technique for detecting the occurrence of the splice edit point is described in U.S. Ser. No. 169,093, filed July 15, 1980. In this technique, a control signal is recorded, and the phase of this control signal is sensed during a playback operation. If the relative phase of the control signal is delayed or advanced with respect to its expected phase, the edit splice point is indicated. Unfortunately, there is a limit on the precision of this control signal phase detection technique such that the precise location of the splice point might not be detected with as high a degree of accuracy as can be attained by the present invention.
Another technique that has been proposed for detecting the location of a splice point is to sense the high occurrence of errors in the reproduced data signals. Since there is a high probability of error, and thus a high frequency of occurrence of errors at the splice point, it had been thought that this phenomenon can be turned to account in detecting the location of the splice point. However, it often is difficult to discriminate between errors which occur at a splice point or errors which occur at a high rate because of drop-out, fingerprints, dust, and the like. Consequently, the use of an error-rate detection technique often may provide false indications of the location of a splice point.